1. Field of the Invention
This invention pertains to a method and apparatus for thermally treating a photosensitive element, and particularly to a method and apparatus wherein the thermal treating is provided by vibrationally-induced frictional energy to create a printing form from the photosensitive element.
2. Description of Related Art
Flexographic printing plates are well known for use in printing surfaces which range from soft and easy to deform to relatively hard, such as packaging materials, e.g., cardboard, plastic films, aluminum foils, etc. Flexographic printing plates can be prepared from photosensitive elements containing photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a photopolymerizable layer interposed between a support and a coversheet or multilayer cover element. Upon imagewise exposure to actinic radiation, photopolymerization of the photo-polymerizable layer occurs in the exposed areas, thereby curing and rendering insoluble the exposed areas of the layer. Conventionally, the element is treated with a suitable solution, e.g., solvent or aqueous-based washout, to remove the unexposed areas of the photopolymerizable composition layer leaving a printing relief which can be used for flexographic printing. However, developing systems that treat the element with a solution are time consuming since drying for an extended period (0.5 to 24 hours) is necessary to remove absorbed developer solution.
As an alternative to solution development, a “dry” thermal development process may be used which removes the unexposed areas without the subsequent time-consuming drying step. In a thermal development process, the photosensitive layer or the photopolymerizable layer, which has been imagewise exposed to actinic radiation, is contacted with an absorbent material at a temperature sufficient to cause the composition in the unexposed portions of the photosensitive layer to soften or melt and flow into an absorbent material. See U.S. Pat. No. 3,060,023 (Burg et al.); U.S. Pat. No. 3,264,103 (Cohen et al.); U.S. Pat. No. 5,015,556 (Martens); U.S. Pat. No. 5,175,072 (Martens); U.S. Pat. No. 5,215,859 (Martens); and U.S. Pat. No. 5,279,697 (Peterson et al.). The exposed portions of the photosensitive layer remain hard, that is do not soften or melt, at the softening temperature for the unexposed portions. The absorbent material collects the softened un-irradiated material and then is separated or removed from the photosensitive layer. The cycle of heating and contacting the photosensitive layer may need to be repeated several times in order to sufficiently remove the flowable composition from the un-irradiated areas and form a relief structure suitable for printing. After such processing, there remains a relief structure having raised features of irradiated, hardened composition that represents the irradiated image.
Processors for thermal development of flexographic printing elements are known. U.S. Pat. No. 5,279,697 describes an automated process and apparatuses for handling an irradiated printing element and accomplishing heating and pressing to remove the unirradiated composition from the element. One embodiment of the thermal development apparatus includes the heated plate that is brought into intimate contact with an absorbent material which in turn contacts an upper surface of a flexible (photosensitive) sheet that resides on a base. The heated plate remains in place for a sufficient time to liquefy a portion of polymer material in the sheet and allow the liquefied polymer material to be absorbed onto the absorbent material. The heated plate is removed and the flexible sheet and the absorbent material advance together while rolls separate the absorbent material from the formed flexographic sheet.
U.S. Pat. No. 5,279,697 describes another embodiment of an automated process and apparatus for handling an irradiated printing element and accomplishing repeated heating and pressing to remove the unirradiated composition from the element. U.S. Pat. No. 6,797,454 B1 also describes a method and apparatus for thermal processing a photosensitive element. In both thermal processing apparatuses the absorbent material is a continuous sheet of a web, typically a non-woven, which is passed over a hot roll. The hot roll is urged towards a drum carrying the photosensitive element pressing the web against the photosensitive element and forming a nip. Heat is transferred by conduction from the hot roll, through the absorbent web, to the photosensitive element upon contact so the temperature of the composition layer is raised sufficiently to enable the unirradiated portions of the composition layer to liquefy and be absorbed into the absorbent web. As the drum and hot roll rotate in contact together, the web is pressed against the photosensitive element to absorb the liquefied unirradiated composition and is then separated from the element.
It is known that the thermal process has productivity and environmental advantages over the solvent treatment process. However, in the existing thermal process, both the photosensitive layer and the support are heated to soften or melt the unirradiated or uncured material for absorption by the absorbent material. Stated another way, in the existing thermal process the entire photosensitive element including the photopolymerized (i.e., cured) portions and the non-polymerized (i.e., uncured) portions of the photosensitive layer, as well as the support, are heated simply to cause the softening of the non-polymerized portions. Heating of the polymerized portions and the support can at times detrimentally impact the quality of the resulting printing form.
In particular, heat can distort the support. With repeated cycles of heating and contacting of the absorbent material to the photosensitive element, the temperature of the support can increase to a temperature higher than its glass transition temperature. Any non-uniform strains induced in the structure of the photosensitive element while the element including the support is hot result in deformations that remain after the element has cooled or returned to room temperature. The deformations are waves of localized distortions resulting in a non-planar topography of the photosensitive element. In addition, it is common that supports for photosensitive elements have shrinkage characteristics when heated that are non-uniform in the x and y planar directions. A problem presented by distortion and non-uniform shrinkage of the support is that for three- and four-color printing processes, three or four plates must be made with images that register exactly for the different colors to print together and produce an accurate final image. If one of the printing forms distorts or shrinks in one direction and the other distorts or shrinks in another direction, the images will not register correctly to produce a quality color print.
Also in the existing thermal treatment methods, the thermal energy is delivered by infrared radiation and/or conduction transfer. Infrared radiation heats the top surface of the photosensitive element and does not selectively heat the non-polymerized portions. Heating by conduction transfer via a hot roll carrying the absorbent material to the top surface of the photosensitive element can be inhibited by the insulating nature of the absorbent material. Heating by conduction transfer from the back of the photosensitive element, that is, from the support side, heats the entire photosensitive element including the support which is prone to distortion and shrinkage at the temperature necessary for softening of the non-polymerized portions of the photosensitive layer.
In addition due to thermal mass of the roll/s, the roll or rolls that are used for conduction heating of the top surface of the photosensitive element take a long time to heat-up and reach the development temperature. The thermal mass of the hot roll/s also impacts the ability to rapidly change the development temperature during processing, particularly from one cycle of heating and contacting to the next, since it takes time for the hot roll/s to reach a different development temperature.
Thus, there is a need for thermal treatment of photosensitive elements that is capable of providing localized and targeted heating of the photosensitive element. It is desirable for thermal treatment to heat only the surface of the photosensitive element to accomplish softening of the non-polymerized portions of the photopolymerizable layer, and avoid heating the support to minimize distortion and shrinkage of the printing form.